In 1834 J. Peltier discovered that when an electric current passes through a junction formed by two different metals a temperature change across the junction results, and that one metal will be heated while the other is cooled. If the current is reversed, the first material will be cooled and the second material heated. Devices relying on the so-called Peltier effect are used in many applications for equipment temperature control, including refrigerators and cooling devices for microelectronic equipment.
Modern Peltier devices are typically composed of a plurality of alternate segments of heavily doped n-type and p-type semiconductors which are electrically connected in series and thermally connected in parallel. Peltier coolers are typically connected between the equipment to be cooled and a heat sink. Such devices are described, for example, in U.S. Pat. No. 4,929,282 to Brun et al. and in U.S. Pat. No. 5,448,109 to Cauchy, both of which are incorporated herein by reference.
Bismuth telluride (BiTe) is a popular choice of material for the construction of Peltier devices because the material is a semiconductor that is both a good conductor of electricity and a poor conductor of heat. In a typical application, alternate pairs of p-type and n-type regions of bismuth telluride are created by heavy doping.
A typical Peltier thermoelectric cooler consists of pairs of p-type and n-type material connected in series and sandwiched between two closely spaced ceramic plates. When connected to a DC power source, current flow through the series of p-n junctions causes heat to move from one side of the thermoelectric cooler to the other. In a typical application, the cold side of the thermoelectric cooler is connected to the equipment to be cooled while the hot side is connected to a heat sink for carrying the heat away. If the direction of current is reversed, however, the thermoelectric device can operate as a heater. The maximum power that a single practical thermoelectric cooler device can pump is about 125 Watts. However, multiple thermoelectric cooler devices can be used in a given cooler application if required.
As described above, both heating and cooling operations by a single Peltier device can be performed by controlling the magnitude and direction of a direct current flowing through the device. A Peltier device can thus be used to control the operating temperature of equipment by detecting the temperature of the equipment and controlling the current to the Peltier device mounted to the equipment accordingly. The overall performance of a Peltier device in controlling the temperature of the equipment is related to the efficiency of the thermal coupling between the Peltier device and the equipment to be controlled.
Peltier coolers are utilized in conjunction with a variety of electrical devices, including laser devices, to dispose of heat generated during the operation of the electrical device. For example, U.S. Pat. No. 5,515,682 to Nagakubo et al. ("Nagakubo") discloses a laser diode mounted on a Peltier device, with the Peltier device controlling the operating temperature of the laser diode through temperature feedback. According to Nagakubo, the small laser diode is mounted on the Peltier device, which is in turn mounted within a hybrid microelectronic package providing bias current for the diode laser and control circuitry for the Peltier temperature control device.
However, some electrical devices, including some laser devices, require even more precise temperature control than the temperature control required by a laser diode. For example, the temperature of a microchip laser must typically be maintained within a few degrees Fahrenheit in order to provide a laser output varying in frequency by less than two percent. The increased temperature control requirements of a microchip laser impose accordingly greater demands on the associated heat removal system.
A microchip laser is a single-crystal bulk laser device that emits coherent radiation upon stimulation by an external light source. For example, a neodymium yttrium aluminum garnet (Nd:YAG) crystal may be excited by an external pump laser source to deliver coherent radiation at fairly high power levels. Relative to other high power laser sources, such as gas lasers or semiconductor lasers, a microchip laser has the advantages of small size, simple construction, low cost and no need for electrical bias current.
High power microchip lasers, such as high power Nd:YAG microchip lasers, are designed to produce output pulses having a high peak output power, such as 10-30 kilowatts or more for a 1-100 kHz diode pumped Nd:YAG microchip laser. These high levels of output power are required in a number of applications, including laser radar, welding, materials processing, surface coating, isotope separation and x-ray lithography, among others. In order to obtain such high power levels, a primary laser, such as a microchip laser, can be pumped by a laser pump source.
In generating pulses having a relatively high average or high peak output power and a relatively high repetition rate, a microchip laser generates a significant amount of heat in a small space. The heat generated may be relatively large and may thereby elevate the temperature of the microchip laser, if not properly removed. The heat generated by a laser is inversely proportional to the optical pumping efficiency of the laser and may be calculated as P.sub.OUT /P.sub.ABS, where P.sub.ABS =P.sub.IN (1-e-.alpha.x) where .alpha. is the absorption coefficient of the microchip laser active region material and x is the length of the microchip laser active region material. Typically, the power generated as heat by a conventional microchip laser is approximately 85 percent of the optical power delivered to the microchip laser.
It is important that an effective means to remove heat from the microchip laser device be provided so as to avoid degrading the microchip laser by prolonged excessive temperature exposure. Moreover, temperature control of a high powered microchip laser is important for applications requiring precise laser output control. For example, the output frequency of a microchip laser can vary as a function of the temperature of the microchip laser. In applications where a nearly constant laser output pulse frequency is required, precise temperature control of the microchip laser is important.
A number of heat sinking approaches for the efficient removal of heat from microchip lasers have been deployed, such as by directly mounting or bonding a metallic or ceramic heat sink to the microchip laser and providing a thermal path for heat removal. Such cooling approaches can effectively remove heat, but are unable to provide the temperature control necessary for certain laser applications requiring precise control of output frequency or other laser operating parameters. Additionally, most conventional microchip laser assemblies utilize the same heat sink structure for both the microchip laser and the pump source (for example, the laser diode). Even though the microchip laser and the pump source may have significantly different thermal requirements and produce heat at different rates, conventional microchip laser assemblies typically do not provide for independent temperature control of the microchip laser and the pump source, thereby precluding individual optimization of the temperature of the microchip laser and the pump source.